use warnings;
use strict;

my $contact = "Firstname Lastname";
$contact =~ s/ .* \s (\S+) $ /Mr. $1/x;
print "$contact\n";
[download]

Or maybe Lingua::EN::NameParse

Your regexp is too "expensive". You do not need capturing.

Simply delete everything up to a whitespace:

use warnings;
use strict;

my $contact = "Firstname Lastname";
$contact =~ s/^.*\s/Mr. /;
print "$contact\n";
[download]

---

s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

The trouble with this sort of problem statement is that it is subject to all kinds of counter-exemplification: so, what about a name like "G. Gordon Liddy Sr., PhD"?

Exactly. I don't know how to handle that, but I did do this and it worked okay:

@contactname = split(' ',$contact);
$firstname = $contactname[0];
$lastname = $contactname[1];
$showcontactname = "Mr. $lastname";
[download]

While I agree with your statement that ..The trouble with this sort of problem statement is that it is subject to all kinds of counter-exemplification I will want to second toolic mention of Lingua::EN::NameParse.

And for a one off name like you mentioned one could do this:

print sprintf 'Mr. %3$s' => split/ / => "G. Gordon Liddy Sr., PhD"; #M
+r. Liddy
[download]

If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

While I understand what the fat commas are doing in there (I think), it's more perplexing why you used print sprintf instead of just printf. What am I missing here?

 [metacpan://parse name first last], [cpan://parse name first last] parse name first last, parse name first last

 [metacpan://first last name], [cpan://first last name] first last name, first last name

Locale::ID::GuessGender::FromFirstName - Guess gender of an Indonesian first name
Lingua::Names - test a string to see if it is a first name
Text::GenderFromName - Guess the gender of an American first name.
Data::RandomPerson::Names::ArabicLast - A list of last names
Data::RandomPerson::Names::EnglishLast - A list of last names

stemming
Lingua::Stem - Stemming of words
Text::English - Porter's stemming algorithm
SWISH::Stemmer - Perl extension for stemming words using a modified version of the Stem routine from the SWISH search engine.

so the idea is, split on whitespace (get words), get "stem", lookup parts in dictionary, decide from there

 
use warnings;
use strict;

my $contact = "Firstname Lastname";
#my $contact = "G. Lastname";

my ($Fname,$Lname) = split(' ',$contact);

if($Fname =~ /.*\./ )
{
        print join (' ', ('Mr',$Fname,$Lname));
}
else
{
        print join (' ', ('Mr',$Lname));
}
[download]

/.*\./ is equivalent to /\./